1. Field of the Invention
The invention relates to a method of manufacturing a flash memory device, and more specifically, to a method forming a dielectric film formed between a floating gate and a control gate of a flash memory device.
2. Related Technology
Generally, the gate of the flash memory device has a structure including a tunnel oxide film 11, a floating gate 12, a dielectric film 13 and a control gate 14, all of which are formed on a silicon substrate 10, as shown in FIG. 1. Reference numeral 15 indicates an isolation film.
The program, erase and read operations of the flash memory device constructed above are performed in such a manner that electrons are injected into or drawn from the floating gate 12 by applying an adequate bias voltage to the control gate 14 and the silicon substrate 10.
The dielectric film 13 has an oxide-nitride-oxide (ONO) structure in which a first oxide film 13a, a nitride film 13b and a second oxide film 13c are stacked. Of them, the first and second oxide films 13a and 13c are formed by means of a dichlorosilane (SiH2Cl2)-based chemical vapor deposition (CVD) method. The CVD oxide film formed by CVD, however, has a lower film quality than an oxide film formed by means of a common dry and wet oxidization method.
The dielectric film 13 greatly affects the operation of the flash memory device. More particularly, a thickness and film quality of the first oxide film 13a has a significant influence on charge leakage and retention characteristics of the flash memory device.
In the design rule of 70 nm or less, a distance between the floating gates 12 is very narrow, i.e., 10 nm or less. If the dielectric film 13 is formed to have an existing thickness, voids A are caused in the dielectric film 13 since between-the-floating gates 12 are not fully filled at the time of subsequent deposition of polysilicon for control gate. These voids A generate poly residue between cells in a subsequent gate etch process, resulting in generation of bridges among the cells.
In order to prevent generation of these voids, it is necessary to thinly deposit the dielectric film 13 because a space between the floating gates 12 has to be secured. If the thickness of the first oxide film 13a is reduced, however, there is a problem in that a data retention characteristic is degraded.
In order to form an oxide film having a good data retention characteristic and a uniform and thin thickness, it is preferred that a thermal oxidation process is employed.
A thermal oxidization process is usually performed at an atmospheric pressure in which a hydrogen (H2) gas and an oxygen (O2) gas are sufficiently supplied in order to induce a spark reaction. In the case where this thermal oxidization process is practiced on a silicon substrate that is not processed, an oxide film having good data retention characteristic and a thin thickness can be formed. If the thermal oxidization process is practiced on the floating gate 12 made of doped polysilicon not the silicon substrate that is not processed, excessive oxidization is generated along the grain boundary of the floating gate 12, which is thermally unstable. It is thus difficult to form an oxide film having a good data retention characteristic and a uniform and thin thickness.